1. Field of the Invention
The present invention relates to a liquid injector for injecting a liquid into a subject, and more particularly to a liquid injector for injecting a liquid into a subject who is to be imaged by an imaging diagnostic apparatus such as a CT (Computed Tomography) apparatus, an MRI (Magnetic Resonance Imaging) apparatus, an angiography apparatus, or the like.
2. Description of the Related Art
Presently available imaging diagnostic apparatus for capturing fluoroscopic images of subjects include CT scanners, MRI apparatus, PET (Positron Emission Tomography) apparatus, ultrasonic diagnostic apparatus, CT angiography apparatus, MR angiography apparatus, and ultrasonograph.
When such an imaging diagnostic apparatus is used to capture a fluoroscopic image of a subject, it is occasionally practiced to inject a liquid such as a contrast medium or a saline solution into the subject. There have been liquid injectors in practical use for automatically injecting a liquid into a subject (see, for example, patent documents 1, 2 below).
Patent document 1: Japanese laid-open patent publication No. 2002-11096;
Patent document 2: Japanese laid-open patent publication No. 2002-102343.
Such a liquid injector has a liquid injection mechanism comprising a drive motor and a slider mechanism, and employs a liquid syringe that is removably mounted on the liquid injection mechanism. The liquid syringe comprises a cylinder and a piston slidably inserted in the cylinder. The cylinder is filled with a liquid, e.g., a contrast medium, to be injected into the subject.
The liquid syringe is connected to the subject by an extension tube and set on the liquid injection mechanism. The liquid injection mechanism individually holds the piston and the cylinder and moves them relatively to each other for injecting a liquid, typically a contrast medium, from the liquid syringe into the subject. If the operator of the liquid injector enters the data of an injection rate for the contrast medium into the liquid injector, then the liquid injector injects the contrast medium into the subject at the injector rate whose data have been entered into the liquid injector. The injected contrast medium changes the image contrast of the subject, allowing the imaging diagnostic apparatus to capture a good fluoroscopic image of the subject.
Thus, the liquid injector can inject a contrast medium into a subject to be imaged by an imaging diagnostic apparatus in order to change the image contrast of the subject so that the imaging diagnostic apparatus is able to capture a good fluoroscopic image of the subject.
There has been conducted an experiment in which a contrast medium for CT was actually injected into a subject by a liquid injector and a CT value representing an image contrast was measured. The results of the experiment have indicated that, as shown in FIG. 11 of the accompanying drawings, the CT value increased nonlinearly and then quickly decreased, and remained at an optimum level only for a short period of time. Therefore, even when a contrast medium is injected into a subject using the conventional liquid injector, it is difficult for the imaging diagnostic apparatus to capture a good fluoroscopic image of the subject.
Moreover, as shown in FIG. 11, the CT value has its peak level much higher than the optimum level, indicating that the subject is dosed with an excessive amount of contrast medium. Consequently, the contrast medium is consumed too much, and tends to adversely affect the subject's health. Simply reducing the amount of contract medium to be injected into the subject would not solve the problem because the period of time in which the CT value approximates the optimum level would be reduced, making it more difficult to capture a good fluoroscopic image of the subject.